The goal of the proposed research is to understand the molecular bases for regulation of immunoglobulin (heavy and light chain) biosynthesis. In vivo translation analysis of wild type and variant cells under different physiological conditions should help to define translational controls. The occurrence and properties of H and L chain mRNA in the nucleus and cytoplasm will be compared in wild type and variant cells. Using a cDNA probe complementary to H and/or L chain mRNA sequences, it may be possible to quantitate the gene dosage of both H and L chains in a given cell. These experiments should contribute to our understanding of the mechanisms of allelic exclusion, amplification, and coordination of H and L chain synthesis in cells which synthesize immunoglobulin. My recent finding that a kappa L chain constant region fragment is synthesized at identical relative rates in all MPC 11 cells suggests that this product may be synthesized de novo. Experiments are proposed to determine whether or not the constant region fragment gene is identical to the gene coding for the constant region (carboxy-terminal) half of the L chain synthesized by the same cell. These studies are particularly relevant to the unique problems posed by the two gene-one polypeptide and allelic exclusion hypotheses.